In cable TV systems, also known as Community Access Television (CATV) systems, a return path is often provided to allow signals from the subscriber equipment to be sent to the central control point, or head-end. The cable return path is typically used to allow subscribers to request movies by entering a request through their cable television receiving equipment (settop). The basic return service for purchasing movies is commonly referred to as "pay per view". Because the CATV system offers a high bandwidth connection to the subscriber, there is the potential for providing a number of other services including video on demand,(VOD) in which a subscriber can instantaneously request, view, rewind, and fast-forward a movie. Personal computers can also be connected to the cable plant and data transmitted from the subscriber location to the head-end. In these applications the signaling from the subscriber equipment to the head-end is generally in the form of a packet of information. In the event that the packet is not received correctly, retransmission can be requested from the subscriber via a retransmission protocol communicated from the head-end to the subscriber equipment in the downstream link.
Telecommunications services can be provided by placing a transmitter and receiver with an appropriate line circuit to drive the telephone at the subscriber location. Such equipment can be located on the side of the subscriber's residence or in the basement of an apartment building or business location and is referred to herein as a coaxial termination unit. Services which would be provided by the coaxial termination unit include Plain Old Telephony service (POTs), advanced telephony services such as Integrated Services Digital Network (ISDN), videotelephony, and high speed data services including Asynchronous Transfer Mode (ATM). In these applications no retransmission protocol is utilized because of the high total data rate involved and the requirement for low network delay; it is therefore necessary to provide a low error rate connection both to and from the subscriber. A typical measure of the performance of the connection is the ratio of the number bits received in error to the number of correctly received bits, referred to as the bit error ratio or BER. For telecommunications links a BER of .ltoreq.10.sup.-10 is the performance goal.
Because the configuration of the cable system is multipoint-to-point from the subscribers to the head-end, the return path has the undesirable characteristic of accumulating or "funneling" noise towards the head-end. The number of subscribers connected to the network is typically greater than 500, and many subscribers can have power dividers (splitters) installed in their homes to allow connection of multiple settops to the cable network. The result of the large number of subscribers and the multiple connections in the home is that there are a large number of points on the cable network where undesirable signals can enter the return path. The commonly used term for undesirable signals on the cable return path is ingress. Ingress is typically AM shortwave broadcast signals and industrial and atmospheric noise, which can enter on the drop cable connecting the subscriber to the cable plant connection termed the tap, and via the coaxial wiring in the subscriber residence or business location. The coaxial wiring used in the home may be of low quality, and will allow ingress because of the low amount of shielding provided with respect to high quality coaxial cable which has a dense braided wire shield which provides high isolation of the center conductor from external electromagnetic fields. The coaxial wiring in the home is also typically unterminated, and can act as an antenna since currents generated on the outside of the shield can to some extent couple to the inside of the shield at the unterminated end and subsequently excite the center conductor. The accumulation of noise on the return path has adversely limited the use of the return path for many purposes.
In some hybrid cable systems lasers are used to transmit signals over an optical fiber to an intermediate location (a node) where the optical signal is converted to an electrical signal and transmitted to the subscriber on coaxial cable. Such a configuration is commonly referred to as a hybrid fiber-coax system. A similar hybrid fiber-coax configuration is used in the return path, where signals from the subscriber are transmitted in a low frequency band (e.g. 5-30 MHz) on the coaxial cable, and subsequently modulated onto a laser for transmission from the node to the head-end via optical cable. In hybrid fiber-coax transmission systems, the noise in the return path is a concern not only because of the resulting degradation in signal-to-noise or signal-to-interference ratio, but because when the total noise is significant the laser may be overmodulated, with the result being that the laser momentarily shuts off or "clips". Because additional communications services are being planned which will utilize the return path, the amount of information the return laser will be required to transmit will increase, and it is possible that clipping in the return path laser will become a limitation in the return path system performance. Use of a lower degree of optical modulation can have economical disadvantages since in order to maintain the same signal-to-noise ratio on an optical link, lowering the degree of optical modulation will require the use of a more powerful and hence more expensive laser. It is thus important to limit the amount of noise in the return path to reduce the noise power to the laser, as well as to maintain a high signal-to-noise and signal-to-interference ratio.